A martensitic stainless steel that is excellent in the mechanical strengths such as a yield strength, a tensile strength and a toughness is also excellent in corrosion resistance and heat resistance. Among the martensitic stainless steels, a martensitic stainless steel containing about 13% Cr, such as 420 steel in AISI (American Iron and Steel Institute), is excellent in corrosion resistance especially under an environment exposed to carbon dioxide gas. The martensitic stainless steel containing about 13% Cr is generally called as “13% Cr steel”.
However, this 13% Cr steel has a lower maximum temperature that is applicable for practical use. Therefore, exceeding the lower maximum temperature gives a less corrosion resistance, which may result in restricting the applicable field of use of this 13% Cr steel.
In this context, another martensitic stainless steel has been improved by adding an Ni element to the 13% Cr steel. This improved martensitic stainless steel is generally called as “super 13Cr steel”. The improved martensitic stainless steel has not only higher mechanical strength such as a yield strength, but also better corrosion resistance for hydrogen sulfide, as compared with the 13% Cr steel. Then, this super 13Cr steel is particularly suitable for an oil well tube in an environment containing a hydrogen sulfide.
In manufacturing the improved martensitic stainless steel, a method has been adopted in order to induce a martensite transformation during quenching the steel from a temperature of not less than the AC3 point, followed by tempering. Excessive high mechanical strength is not preferable because higher mechanical strength steel is more susceptible for a sulfide stress cracking. The quenching leads to a martensite structured steel having an excessively high strength, but the subsequent tempering adjusts it to a structured steel that has the desired mechanical strength.
Several methods of manufacturing a martensitic stainless steel in which tempering process was improved to adjust mechanical strength are disclosed as described below.
Japanese Patent Unexamined Publication Nos. 2000-160300 and 2000-178692 disclose a method of manufacturing a high Cr alloy with a low carbon for oil well tube, which has an improved corrosion resistance or stress corrosion cracking resistance with 655 N/mm2 (655 MPa) grade yield strength. The method is as follows: heat treatment of austenitizing, cooling, first tempering at a temperature not less than AC1 point and not more than AC3 point, cooling, and second tempering at a temperature that is not less than 550° C. and not more than AC1 point.
Also, Japanese Patent Unexamined Publication No. H08-260050 discloses a method of manufacturing a martensitic stainless steel seamless steel tube, in which a steel is tempered at a temperature that is not less than AC1 point and not more than AC3 point, and then cooled in order to perform a cold working so that the steel is adjusted to have a desired yield stress.